This invention relates to a liquid container which includes a probe part of a liquid gauge of the type for electrically detecting the quantity of liquid existing in the container by utilizing a variation in the capacitance between two metal plates vertically disposed in the container. For example, the liquid container is useful as a fuel tank in a vehicle.
As is known, it is possible to detect the quantity of a liquid existing in a container by fixedly and vertically disposing two flat and parallel electrode, or a cylindrical electrode and a concentric electrode in the container and measuring capacitance between these two electrodes. This structure is based on the fact that most liquid materials have dielectric constants considerably different from the dielectric constant of air. Assuming that the two electrode plates are opposite to each other in a surface area S with a definite distance d therebetween and that the electrode plates are submerged in the liquid by a fraction (x) of the effective surface area S, the capacitance C between these two plates is given by the following equation: ##EQU1## where E.sub.L is the dielectric constant of the liquid, and E.sub.A is the dielectric constant of air.
As the liquid level in the container rises and falls, the submerged fraction x of the electrode surface area S increases or decreases with corresponding increases or decreases in the value of the capacitance C. By suitably selecting the shape of the electrode plates, it is possible to establish a practically linear relationship between the liquid level or the quantity of liquid existing in the container and the capacitance C since there is a linear relationship between x and C in the above equation.
In practical liquid gauges based on the above described principle, it is popular to utilize the variation in the capacitance (C) to vary the frequency (f) of a pulse signal generated by an oscillator circuit including a resistance R connected in series with the capacitor constituted of the two electrode plates to determine a time-constant. In that case, the relation between the capacitance C and the frequency f is expressed in the following way: ##EQU2## By utilizing the variation in the frequency f of the pulse signal, the liquid level in the container is indicated on a meter which may be either of an analog or digital type.
When a liquid container of the described type is in a stationary state there is little problem in detecting the liquid level by this method. However, the situation is different when the liquid container is installed as a fuel tank of a vehicle such as an automobile. In this case difficulties arise in accurately detecting the quantity of fuel existing in the fuel tank during running of the vehicle because the fuel tank inclines to various degrees and the fuel makes inclining and/or undulating movements and even undergoes an unbalanced displacement toward one side by the action of a centrifugal force when the vehicle rounds a curve on the road. The difficulties are augmented by rapid and great changes in the fuel level attributed to the fact that the fuel tank is shallow relative to its lateral widths and usually has an asymmetric shape in plan view. Attempts to solve such difficulties have involved increasing the effective surface area S of the electrode plates to decrease the amount of a change in the capacitance C with a given change in the fuel level in the tank and/or by disposing at least one additional set of electrode plates at suitably selected location(s) in the fuel tank and connecting all sets of the electrode plates in parallel with one another to thereby compensate irregular changes in the fuel level.
However, these countermeasures have caused additional problems from a practical point of view. Firstly, as the effective surface area of the two oppositely arranged electrode plates is greatly enlarged it becomes difficult to accurately maintain a predetermined distance between these two plates over the entire surface area against the undulating movements of the fuel and the aforementioned centrifugal force. Secondly, assembling of the fuel tank becomes troublesome and requires increased manhours as the surface area and/or total numbers of electrode plates increases. A further matter for serious consideration is how to give mechanical support to the electrode plates. It is usual to fasten each set of two electrode plates to one of the baffleplates, which are fixedly disposed in the fuel tank so as to divide the interior of the tank into several sections for the purpose of suppressing undulations of the fuel during running of the vehicle thereby preventing the fuel from producing noise. However, this structure is not favorable because it requires a large number of parts and many manhours, and resulting in a considerable increase in the total weight of the fuel tank.